On-press developable negative-working lithographic offset printing plates are known in the prior art. For example, U.S. Pat. No. 5,569,573 teaches lithographic printing plates comprising a laser imaging layer containing microencapsulated oleophilic materials in hydrophilic polymer binders.
EP 0 770 495 teaches lithographic printing plates comprising near infrared absorbing materials, polymer binders and thermoplastic particles capable of coalescing under heat.
U.S. Pat. No. 6,983,694 teach on-press developable negative-working offset printing plates coated with near infrared sensitive coating compositions comprising thermoplastic polymeric particles, such as polystyrene or poly(acrylonitrile-co-styrene) particles, non-reactive hydrophilic polymer binder and near infrared absorption dyes.
U.S. Pat. No. 6,261,740 teaches negative-working offset printing plates coated with near infrared sensitive coating compositions containing methoxymethacrylamide copolymers, phenolic resins, iodonium salts and near infrared absorption dyes.
U.S. Pat. Nos. 6,124,425 and 6,177,182 teach on-press developable negative-working offset printing plates coated with thermally reactive near-infrared absorbing copolymers, which undergo cross-linking reactions via cationic polymerization upon exposure to near infrared radiation. The near infrared chromophoric moieties are functionalized to the polymeric backbone via ether and ammonium bonds. These near infrared absorbing polymers are prepared as homogenous solutions.
U.S. Pat. No. 6,960,422 teaches to prepare negative-working offset printing plates, which contain a near infrared sensitive base-coat compositions comprising molecular near infrared dyes, radical generators, radical polymerizable urethane compounds, reactive polymer binders and other additives.
EP patent No. 1234662 teaches to prepare negative-working offset printing plates, the coating compositions of which contain a near infrared absorber, onium initiators and urethane polymers comprising poly(ethylene oxide) side chains. These urethane polymers do not absorb near infrared radiation.
U.S. Pat. Nos. 6,969,575 and 7,001,704 teach on-press developable negative-working offset printing plates having an image-forming layer, which comprise near infrared absorbing microcapsules and acid generator compound. Similarly, U.S. Pat. Nos. 7,001,673 and 7,078,145 teach to prepare on-press developable negative-working offset printing plates. The coating compositions comprise near infrared absorbing microcapsules, which were prepared via emulsification with a high speed homogenizer of the oil phase containing hydrophobic polymers, near infrared absorbers and initiators in hydrophilic polymer water solutions. The preparation of these microcapsules is complicated and these coatings exhibit poor scratch resistance and require an overcoat layer.
U.S. Pat. No. 6,037,102 and EP Application No. 1117005 teach to prepare negative-working photosensitive coating compositions comprising copolymers grafted with poly(ethylene oxide) side chains. These copolymers do not absorb near infrared radiation.
U.S. Pat. No. 6,582,882 teaches to prepare “graft” polymers and copolymers for use in offset printing plates comprising poly(ethylene oxide) side chains, which are grafted on non-crosslinked hydrophobic polymer backbone. These polymers do not absorb near infrared radiation.
U.S. Pat. No. 6,899,994 and co-pending US patent applications No. 2003/0157433; 2003/0064318 and 2005/0123853 teach on-press developable negative-working offset printing plates, which are coated with thermally imageable compositions containing polymer binders, initiator systems and polymerizable components. The described polymer binders are non-crosslinked copolymers having polyethylene oxide and polypropylene block, or graft copolymers co-polymerized with hydrophobic monomers, such as styrene, substituted styrene, alpha-methylstyrene, acrylate ester, methacrylate ester, acrylonitrile, acrylamide, methacrylamide, vinyl halide, vinyl ester, vinyl ether and an alpha-olefin. The polymerizable components are viscous liquid oligomers containing multiple acrylic and methacrylic functional groups. The initiator system contains near infrared absorption dyes and radical producing compounds, such as triazine and iodonium salts.
All of these coating compositions and printing plates show some disadvantages such as having a tacky surface, which causes difficulties for handling and storage, exhibiting phase separation and/or surface crystallization upon storage, requiring high laser power to achieve imaging, having poor substrate adhesion and consequently failing to provide sufficient run length on press, not being developable on-press due to dye staining causing background toning, exhibiting poor scratching resistance, requiring an over-coating layer and/or a special substrate surface treatment and being expensive or difficult to manufacture.
There thus remains a need for new materials and new coatings for lithographic plates that would overcome some or all of the drawbacks of the prior art.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.